1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
Most electronic circuits involve signal conditioning of analog, digital and/or mixed signals. Such signal conditioning produces signals that often undergo further processing including filtering, sampling, amplification and/or digitizing because real world signals contain static or dynamic information both wanted and unwanted. As one example, to maintain a secure link for communications, a frequency hopping technique may be used where the frequency of a transmitter and a receiver changes rapidly, causing noise and/or interference. Because of this frequency change, many communications systems get affected, often making filtering necessary.
Generally, filtering is a process or a mathematical operation of removing an undesired component of a signal while allowing a desired component to pass, e.g., attenuating unwanted frequencies in the signal that may be identified by a spectral analysis. A filter is an electrical circuit that selectively separates a band of signal frequencies, allowing signals in certain frequency ranges to pass through, while attenuating or blocking all other frequencies. A filter provides a pass band, a stop band and a cutoff frequency or corner frequency that defines the frequency boundary between the pass band and the stop band.
For example, to pass a transmit or a receive frequency band of operation in a radio of a base station, one or more band pass or selective filters are generally used. A band pass or selective filter eliminates a selected set of frequencies from a spectrum of a signal, and in the case of a resonant circuit or filter, the band pass or selective filter decrease the level of other frequencies. That is, the resonant circuit or filter responds to frequencies close to a natural frequency much more strongly than to other frequencies. Typically, characteristics of such filters, such as a Q-factor, are fixed and cannot be readily altered. The Q-factor is a measure of a quality of a resonant circuit or filter. For a band pass or selective filter, a difference between an upper cutoff frequency and a lower cutoff frequency is called the bandwidth and the Q-factor is defined as a ratio of a center frequency and a bandwidth. While the center frequency may be a geometric mean between a lower cutoff frequency and an upper cutoff frequency of a frequency band, the bandwidth may be defined as a 3 dB change in level beside the center frequency.
Typically, a radio once manufactured for a specific type of frequency band only works for that specific type of frequency band. To alter frequency bands, one approach proposes use of a bank of different filters with fixed characteristics such that those filters are switched through a matrix. This approach involves more or less a parallel implementation of individual signal paths for each frequency band, which is opposite to the demand for a common signal path for all bands with a frequency agile radio. Although some gains may be obtained through a particular arrangement of the switches, this parallel implementation of individual signal paths for each frequency band adds significant costs to design. One shortcoming entails that every band adds another parallel path and that after manufacturing only an alteration inside a set of bands may be done. Therefore, if a regulation assigns a new frequency band, a tuning of the radio to that new frequency band is difficult, if impossible.
One fundamental problem for flexible or reconfigurable filters is that a technique for altering filter properties degrades the Q-factor, rendering the filters useless because a degraded Q-factor causes the bandwidth to become unacceptably large. However, for an accurate filter it is desirable to have higher Q-factors. A desired Q-factor for a typical filter is normally determined by a ratio of stop-band frequency to a pass-band corner frequency and by an amount of ideal stop-band attenuation. However, conventional filters have a poor Q-factor. The Q-factor and center-frequency of these filters are fixed (or mechanically tuned). Moreover, Q-factors of capacitors and inductors are often too low, especially if they are integrated (Q<100). Further, these Q-factors are typically fixed and cannot be readily varied. In many applications, therefore, one or more additional high-Q resonator filters are used. These high Q resonator filters have an added shortcoming of being very expensive.
As shown in FIG. 2, a prior art technique for improving and tuning a Q-factor uses an Audion receiver. A negative impedance is provided to a resonant circuit, which compensates for the losses, and thus, enhances the Q-factor and narrows the bandwidth. With the Audion receiver, one tube simultaneously serves for amplifying an audio signal and a radio frequency (RF) signal. A part of the amplified RF signal is positively fed back to a resonator in equal phase. One problem with the Audion receiver is that of a high risk of oscillation. That is, because of this oscillation a receiver could become a transmitter. The feedback is not stable since changes in the surroundings of an antenna and a headphone or a loudspeaker detunes the feedback. A feedback knob needs retuning in short intervals to either avoid a low feedback (bad reception) or a high feedback (oscillation). This feedback knob based retuning causes the receiver to fluctuate between a “satisfying” or “catastrophic” behavior.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.